The conventional push-to-talk (PTT) communications system has several transceivers, all tuned to the same channel, namely, the same frequency. Any user who wishes to speak pushes a button on his microphone, causing the transceiver to transmit. Releasing the button causes the transceiver to receive. Any number of users may share the same frequency, provided that there is some way to determine which user is next allowed to transmit.
Trunked radio is a step up from this technology. Instead of sharing a common physical channel, the users share a common logical channel. A user who wishes to start a conversation broadcasts a signal requesting such a start. A controller receives this signal. The controller broadcasts back a signal which allocates a physical channel—a signal which every other user can receive as well. All users then re-tune to the allocated frequency, and the conversation continues as before. Whenever there is a pause in the conversation, the controller can broadcast a new physical channel allocation. Such re-allocation may be required by traffic patterns, signal quality, and the like.
“Trunked” radio gets its name from telephone trunk lines between distant cities. A telephone user in one city may regularly place a call, every day, to another user in the other city. When he does, he places it on a local loop to his local telephone company office, which places it on a trunk to the telephone company office in the other city, which places it on a local loop to the second user. There is no expectation that the same inter-city trunk will be assigned every day. Indeed, the telephone company may change the trunk in the middle of the call (presumably during a pause), without changing either of the local loops.
Trunked radio works as well (or as poorly) in a conference call (more than two users) as it does in a regular call (two users). The controller can do more that just allocate the physical channel. It can also arbitrate which user is next allowed to broadcast. It can alert a pre-selected group of users that a broadcast is being made to them. What it can't do, however, is dynamically change the composition of the group, or give the group geographic coverage driven by the availability of the Internet, or an Intranet or Extranet.